This invention is directed toward corrosion inhibition in evaporative cooling water applications. More particularly, this invention is directed toward the use of azoles inhibitors in combination with high Total Dissolved Solids (TDS), high pH, and low total hardness.
With respect to the inhibition of corrosion for metals benefiting from the discoveries disclosed herein, the most common examples cited are copper and copper alloys because of their good thermal and workability properties for heat transfer and fabrication, and their inherent properties for bio-fouling resistance and corrosion resistant oxide formation under water chemistry conditions addressed by prior art applications. More specifically, these discoveries focus on corrosion inhibitor performance in water chemistry conditions that are the result of evaporative cooling water systems that operate with water chemistry approaching Zero Liquid Discharge (ZLD) in order to conserve water and reduce environmental discharge impact, such as permitted by corrosion and scale inhibition methods recently disclosed by Duke, et al. in U.S. Pat. Nos. 6,929,749; 6,940,193; 6,998,092; and 7,122,148, all of which are herein expressly incorporated by reference. Such method water chemistry may approach or exceed seawater TDS concentration, but seawater applications are typically pH neutral (pH 7.4 to 8.7 in evaporative cooling water) and also contain high hardness concentrations in evaporative cooling water applications.
Corrosive attack of copper by ammonia in water is well known in the water treatment industry, and presents a particular challenge to reuse of wastewater sources that contain ammonia in cooling water systems that use copper and other metals and alloys which are vulnerable to ammonia. Ammonia and ammonium ion are reported to exist in equilibrium as both the ammonium ion and ammonia gas in the pH 7 to 11 range. The equilibrium shifts toward increased ammonium ion concentration as pH approaches 7 and to increased ammonia gas concentration as pH approaches 11. Ammonia gas is volatilized from water by heat, pH elevation and circulating over a cooling tower, typical of ammonia stripper design. With cooling water pH control at greater than pH 9, total ammonia/ammonium ion residuals will be reduced to lower ranges by such tower stripping. Dilute aqueous concentrations of ammonia/ammonium ion (less than 200 mg/L as NH4+) are easily measured by such procedures as Chemetrics test procedure K-1500 which converts and measures the total residual as ammonium ion.
Relative to seawater (high TDS), as reported by Tuthill et al. in Experience with Copper Alloy Tubing, Waterboxes and Piping in MSF Desalination Plants, IDA World Congress on Desalination and Water Reuse, Volume I, Sessions 1 to 3, October 1997, Madrid, Spain, the corrosion resistance of copper nickel alloys depends upon formation of a protective film. Film formation is referred to sometimes as “passivation”. Film formation is affected by pH, time, aeration, velocity, temperature, pollution and other factors. There is generally an inner cuprous oxide film, Cu2O, and an outer cupric oxide, CuO, film. Although cuprous oxides and cupric oxides are the principal components of the films, the lattice usually includes other metallic ions, including iron, nickel, aluminum, calcium, sometimes silicon and other species. Principal anions include chlorides, hydroxides, carbonates, bicarbonates and oxides. There is no fixed composition in these films.
Tuthill also reports that time is a major factor in film formation and also in the degree of protection the film affords. Tuthill referred to studies that have shown that corrosion rates of copper nickel alloys in seawater may gradually decrease over time for periods up to 7 years, with example corrosion rate data for C70600 reduced from approximately 1.9 mpy to about 0.6 mpy over that time frame. Temperature also has a major influence on the rate of film formation. At higher temperatures the film forms and matures faster. At lower temperatures, the film forms and matures more slowly. Another major factor influencing film formation is pH. Tuthill referred to studies that reported on film formation for C70600, C71500 and C68700 alloys in seawater found no film formation below pH 6. The unfilmed corrosion rates were high, of the order of 35 mpy (0.89 mm/yr.). At higher pH, corrosion rates for these metals were reported to be lower at normal rates for seawater. Tuthill also reported corrosion rates for these three copper tubing alloys in seawater systems, depending on chlorination practices, varied from less than 1 mpy to 3.2 mpy. Corrosion behavior in fresh, brackish and higher salinity waters is quite similar to performance in seawater.
Tuthill also reports that ammonia is sometimes encountered in the seawater feed to desalination plants. In the presence of air and ammonia, aluminum brass is subject to stress corrosion cracking. Aluminum bronze is more resistant, while copper nickel alloys are highly resistant to ammonia stress corrosion cracking. Ammonia also tends to increase the general corrosion rates of copper alloys. Copper nickel alloys have been reported to be three orders of magnitude more resistant than aluminum brass.
Nitrogen-containing compound, such as benzotriazoles, are commonly used as antioxidants and corrosion inhibitors for copper and copper alloys in many environments and applications. The lone electron pairs on the nitrogen will coordinate with the metal substrate and will result in a direct attach in the case of cyanate esters, and a parallel attach in the case of triazines, isocyanurates, and blocked isocyanates by physical absorption. The lone electron pairs of the nitrogens further facilitate the coordination of the nitrogen atoms to the Cu substrates in the event any oxidation occurs to form Cu+ or Cu2+ ions or oxides. In the case of the triazines and isocyanurates, the functional group can be any reactive or polymerizable functional group, and preferably is an epoxy, allyl, vinylether, hydroxyl, acrylate or methacrylate group. In the case of the polyfunctional cyanate esters and isocyanates, these groups themselves are homo-polymerizable or are reactive with complementary reactive groups, such as, epoxy, carboxyl, hydroxyl and amine functionalities.
Such nitrogen containing inhibitors, referred to as azoles in the water treatment industry, include the more commonly applied Tolytriazole (TTA), Benzotriazole (BTA), and variations of chemical structure that produce comparable inhibiting films on metal surfaces including 4-(alkyl)substituted benzotriazole and 5-(alkyl)substituted BTA where the alkyl group (CnH2n+1) has n=1 to 18. Note: n=1 for CH3 in tolyltriazole which is a mixture of 4-methylbenzotriazole and 5-methylbenzotriazole. The 5-(n-butyl) benzotriazole which was patented by Betz is an example of n=4 in the n-butyl alkyl group C4H9.
Use of azoles corrosion inhibitor chemistry for metals is known in evaporative cooling water treatment, often being combined with other inhibitors that rely on control of TDS at lower and less corrosive concentrations by blow down wastage of tower water, and control at neutral pH (i.e., pH between 6.0 pH and 9.0) by use of blow down wastage or acid feed for pH control to limit scale formation. Such systems do not typically control hardness at less than 200 mg/L (as CaCO3) as this condition increases water corrosiveness. Azoles inhibitors have also been used to inhibit corrosion of copper by ammonia/ammonium ion in systems operated within prior art control ranges for pH and TDS. Inhibitor use and consumption by blow down wastage is significant, thus these application are costly and environmentally inefficient.
Use of azoles as corrosion inhibitors to protect metals is known in closed system corrosion inhibition applications. Such applications often combine azoles with other inhibitors used in non-evaporative closed loop systems such as engine cooling systems and closed system cooling loops that have minimal water and inhibitor losses. However, such applications of azoles in closed systems operate without high TDS concentrations in the cooling water since there is no evaporation and concentration of makeup water chemistry. As such, inhibitor use and consumption are limited and their application is cost efficient.